Process for forming polyolefins

ABSTRACT

Processes of forming polyolefins are described herein. One or more specific embodiments of the processes generally include introducing olefin monomer selected from C 2 -C 3  olefins into a first reaction zone under first polymerization conditions to form a first polyolefin; withdrawing a transfer effluent from the first reaction zone, the transfer effluent including first polyolefin and unreacted olefin monomer; introducing the transfer effluent, a comonomer selected from C 4 -C 8  olefins, and additional olefin monomer to a second reaction zone under second polymerization conditions to form a second reactor product; maintaining an essentially constant comonomer:olefin monomer ratio in the second reaction zone; and withdrawing at least a portion of the second reactor product, wherein the second reactor product includes a bimodal polyolefin.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND

1. Field of the Invention

The present invention generally relates to processes for formingpolyolefins and controlling characteristics of the formed polyolefins.In particular, embodiments relate to processes for controllingpolyolefin characteristics in multi-modal polymerization processes.

2. Related Art

This section introduces information from the art that may be related toor provide context for some aspects of the techniques described hereinand/or claimed below. This information is background facilitating abetter understanding of that which is disclosed herein. This is adiscussion of “related” art. That such art is related in no way impliesthat it is also “prior” art. The related art may or may not be priorart. The discussion is to be read in this light, and not as admissionsof prior art.

Olefin polymerization processes are well known and numerous methods havebeen disclosed in the literature relating to controlling such reactions.However, real-time control of polymer characteristics has beendifficult. Process variables in the reaction zone can change suddenlyand their effect on the monomer concentration in the reaction zone maynot be detected and quickly addressed. Such delayed monomerconcentration analysis may cause less than desired reaction productionand/or polyolefin property control. Accordingly, there is a need forbetter monomer concentration analysis and control.

The present invention is directed to resolving, or at least reducing,one or all of the problems mentioned above.

SUMMARY

Various embodiments of the present invention include processes offorming polyolefins. The processes generally include introducing olefinmonomer selected from C₂-C₃ olefins into a first reaction zone underfirst polymerization conditions to form a first polyolefin; withdrawinga transfer effluent from the first reaction zone, the transfer effluentincluding first polyolefin and unreacted olefin monomer; introducing thetransfer effluent, a comonomer selected from C₄-C₈ olefins, andadditional olefin monomer to a second reaction zone tinder secondpolymerization conditions to form a second reactor product; maintainingan essentially constant comonomer:olefin monomer ratio in the secondreaction zone; and withdrawing at least a portion of the second reactorproduct, wherein the second reactor product includes a bimodalpolyolefin.

One or more embodiments include the process of any preceding paragraph,wherein the olefin monomer includes ethylene.

One or more embodiments include the process of any preceding paragraph,wherein the comonomer includes hexene.

One or more embodiments include the process of any preceding paragraph,wherein the first reaction zone, the second reaction zone or acombination thereof include a loop slurry reaction vessel.

One or more embodiments include the process of any preceding paragraph,wherein the second reaction zone comprises a loop slurry reaction vesseland the loop slurry reaction vessel comprises a plurality of olefin feedlocations, comonomer feed locations or a combinations thereof.

One or more embodiments include the process of any preceding paragraph,wherein maintaining a comonomer:olefin monomer ratio essentiallyconstant includes determining a concentration of carry over olefinmonomer in the transfer effluent; and adjusting a rate of introductionof the additional olefin monomer into the second reaction zone,adjusting a rate of introduction of the comonomer into the secondreaction zone or a combination thereof in response to the carry overolefin monomer concentration.

One or more embodiments include the process of any preceding paragraph,wherein the carry over olefin monomer concentration is determined by aprocess including irradiating in-situ the transfer effluent; measuringscattered energy from the transfer effluent; and determining from themeasured scattered energy the carry over olefin monomer concentration.

One or more embodiments include the process of any preceding paragraph,wherein the carry over olefin monomer concentration is determined bycalculating the production rate of first polyolefin in the firstreaction zone.

One or more embodiments include the process of any preceding paragraph,wherein the production rate of the first polyolefin in the firstreaction zone is determined from the calculated reaction quotient(Q_(rxn)) and the heat of polymerization per unit of polyolefinproduced.

One or more embodiments include the process of any preceding paragraph,wherein the carry over olefin monomer concentration is determined byperforming an energy balance calculation for the first reaction zone.

One or more embodiments include the process of any preceding paragraph,wherein the bimodal polyolefin includes a first polyethylene fractionhaving an average molecular weight of from 15,000 to 50,000 and a secondpolyethylene fraction having an average molecular weight of greater than100,000.

One or more embodiments include the process of any preceding paragraph,wherein the bimodal polyolefin includes at least 40% first polyethylenefraction.

One or more embodiments include the process of any preceding paragraph,wherein the bimodal polyolefin includes from 40% to 60% of the firstpolyethylene fraction.

One or more embodiments include bimodal polyolefins formed by theprocess of any preceding paragraphs.

One or more embodiments include processes of forming polyolefinsincluding introducing olefin monomer selected from C₂-C₃ olefins into areaction zone under polymerization conditions to form a polyolefin;introducing a comonomer selected from C₄-C₈ olefins into the reactionzone; and maintaining an essentially constant comonomer:olefin monomerratio in the reaction zone by determining a concentration of olefinmonomer introduced into the reaction zone; and adjusting a rate ofintroduction of olefin monomer into the reaction zone, adjusting a rateof introduction of the comonomer into the reaction zone or a combinationthereof in response to the concentration of olefin monomer concentrationintroduced into the reaction zone.

One or more embodiments include the process of the preceding claim,wherein the olefin monomer concentration is determined by a processincluding irradiating in-situ the input stream; measuring scatteredenergy from the input stream; and determining from the measuredscattered energy the olefin monomer concentration.

One or more embodiments include a process of forming polyolefinsincluding introducing olefin monomer selected from C₂-C₃ olefins andhydrogen into a first reaction zone under first polymerizationconditions to form a first polyolefin; withdrawing a transfer effluentfrom the first reaction zone, the transfer effluent including firstpolyolefin and unreacted olefin monomer; introducing the transfereffluent, a comonomer selected from C₄-C₈ olefins, and additional olefinmonomer to a second reaction zone under second polymerization conditionsto form a second reactor product; determining a melt index of the firstpolyolefin in the transfer effluent, the first reaction zone or acombination thereof; correlating density of the first polyolefin withthe melt index of the first polyolefin; and adjusting a rate ofintroduction of the hydrogen into the first reaction zone in response tothe melt index of the first polyolefin and a pre-determined bimodalpolyolefin density; and withdrawing at least a portion of the secondreactor product, wherein the second reactor product comprises thebimodal polyolefin.

One or more embodiments include the process of the preceding claimfurther including separating at least a portion of the transfer effluentto form a lighter stream and a heavier stream; and determining a meltindex of the first polyolefin in the heavier stream.

One or more embodiments include the process of any preceding claim,wherein the heavier stream is introduced into the second reaction zone.

One or more embodiments include the process of any preceding claim,wherein the separating includes passing the at least a portion of thetransfer effluent through a flash tank, a hydrocyclone or a combinationthereof to form the lighter stream and the heavier stream.

One or more embodiments include a process of forming polyolefinsincluding introducing olefin monomer selected from C₂-C₃ olefins andhydrogen into a first reaction zone under first polymerizationconditions to form a first polyolefin; withdrawing a transfer effluentfrom the first reaction zone, the transfer effluent including firstpolyolefin and unreacted olefin monomer; withdrawing a second effluentfrom the first reaction zone; separating at least a portion of thesecond effluent to form a lighter stream and a heavier stream;introducing the transfer effluent, a comonomer selected from C₄-C₈olefins, and additional olefin monomer to a second reaction zone undersecond polymerization conditions to form a second reactor product;determining a melt index of the first polyolefin in the heavier stream;correlating density of the first polyolefin with the melt index of thefirst polyolefin; and adjusting a rate of introduction of the hydrogeninto the first reaction zone in response to the melt index of the firstpolyolefin and a predetermined bimodal polyolefin density; andwithdrawing at least a portion of the second reactor product, whereinthe second reactor product comprises the bimodal polyolefin.

One or more embodiments include the process of any preceding claim,wherein the separating includes passing the at least a portion of thetransfer effluent through a flash tank, a hydrocyclone or a combinationthereof to form the lighter stream and the heavier stream.

One or more embodiments include a process of controlling bimodalpolyolefin density including introducing olefin monomer selected fromC₂-C₃ olefins into a first reaction zone under first polymerizationconditions to form a first polyolefin exhibiting a first density;withdrawing a transfer effluent from the first reaction zone, thetransfer effluent including first polyolefin and unreacted olefinmonomer; introducing the transfer effluent, a comonomer selected fromC₄-C₈ olefins, and additional olefin monomer to a second reaction zoneunder second polymerization conditions to form a second polyolefinexhibiting a second density; withdrawing at least a portion of a secondreactor product from the second reaction zone, wherein the secondreactor product comprises a bimodal polyolefin including the firstpolyolefin and the second polyolefin and exhibiting a bimodal polyolefindensity; controlling the bimodal polyolefin density within a targetdensity by a process including maintaining an essentially constantsecond density within the second reaction zone by maintaining anessentially constant comonomer:olefin monomer ratio in the secondreaction zone; determining a melt index of the first polyolefin in thetransfer effluent, the first reaction zone or a combination thereof;correlating the first density with the melt index of the firstpolyolefin; and adjusting a rate of introduction of the hydrogen intothe first reaction zone in response to the melt index of the firstpolyolefin and the target density.

One or more embodiments include the process of any preceding claim,wherein maintaining a comonomer:olefin monomer ratio essentiallyconstant includes determining a concentration of carry over olefinmonomer in the transfer effluent; and adjusting a rate of introductionof the additional olefin monomer into the second reaction zone,adjusting a rate of introduction of the comonomer into the secondreaction zone or a combination thereof in response to the carry overolefin monomer concentration.

One or more embodiments include the process of any preceding claim,wherein the carry over olefin monomer concentration is determined by aprocess including irradiating in-situ the transfer effluent; measuringscattered energy from the transfer effluent; and determining from themeasured scattered energy the carry over olefin monomer concentration.

One or more embodiments include the process of any preceding claim,wherein the carry over olefin monomer concentration is determined bycalculating the production rate of first polyolefin in the firstreaction zone.

One or more embodiments include the process of any preceding claim,wherein the production rate of the first polyolefin in the firstreaction zone is determined from the calculated reaction quotient(Q_(rxn)) and the heat of polymerization per unit of polyolefinproduced.

One or more embodiments include the process of any preceding claim,wherein the carry over olefin monomer concentration is determined byperforming an energy balance calculation for the first reaction zone.

One or more embodiments include the process of any preceding claim,wherein the bimodal polyolefin includes a first polyethylene fractionhaving an average molecular weight of from 15,000 to 50,000 and a secondpolyethylene fraction having an average molecular weight of greater than100,000.

One or more embodiments include the process of any preceding claim,wherein the bimodal polyolefin includes at least 40% first polyethylenefraction.

One or more embodiments a bimodal polyolefin formed from the process ofany preceding claim.

The above paragraphs present a simplified summary of the presentlydisclosed subject matter in order to provide a basic understanding ofsome aspects thereof. The summary is not an exhaustive overview, nor isit intended to identify key or critical elements to delineate the scopeof the subject matter claimed below. Its sole purpose is to present someconcepts in a simplified form as a prelude to the more detaileddescription set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

The claimed subject matter may be understood by reference to thefollowing description taken in conjunction with the accompanyingdrawings, in which like reference numerals identify like elements, andin which:

FIG. 1 illustrates a schematic of a simplified polyolefin process foruse in polymerization illustrations.

FIG. 2 illustrates a schematic of a specific embodiment of a loopreactor.

FIG. 3 illustrates an essentially linear correlation of density and meltindex for a hypothetical system.

FIG. 4 illustrates a non-linear correlation of density and melt indexfor a hypothetical system.

While the claimed subject matter is susceptible to various modificationsand alternative forms, the drawings illustrate specific embodimentsherein described in detail by way of example. It should be understood,however, that the description herein of specific embodiments is notintended to limit the claimed subject matter to the particular formsdisclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

Illustrative embodiments of the subject matter claimed below will now bedisclosed. In the interest of clarity, not all features of an actualimplementation are described in this specification. It will beappreciated that in the development of any such actual embodiment,numerous implementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a developmenteffort, even if complex and time-consuming, would be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

In the description below, unless otherwise specified, all compoundsdescribed herein may be substituted or unsubstituted and the listing ofcompounds includes derivatives thereof. Further, various ranges and/ornumerical limitations may be expressly stated below. It should berecognized that unless stated otherwise, it is intended that endpointsare to be interchangeable. Further, any ranges include iterative rangesof like magnitude falling within the expressly stated ranges orlimitations.

Embodiments described herein include processes of forming polyolefins(i.e., polymerization processes). As known in the art, olefinpolymerization processes include contacting an olefin monomer with acatalyst system within a reaction zone to form the polyolefin. Catalystsystems may include any catalyst system(s) useful for polymerizingolefin monomers. For example, the catalyst systems may be selected fromchromium based catalyst systems, single site transition metal catalystsystems including metallocene catalyst systems, Ziegler-Natta catalystsystems and combinations thereof, for example. As known in the art thecatalysts may be activated for subsequent polymerization and may or maynot be associated with a support material for example.

Once the catalyst system is prepared, as described above and/or as knownto one skilled in the art, a variety of polymerization processes may becarried out using that catalyst system. The polymerization conditions(e.g., equipment, process conditions, reactants, additives and othermaterials used in polymerization processes) will vary in a givenprocess, depending on the desired composition and properties of thepolymer being formed. Such processes may include solution phase, gasphase, shiny phase, bulk phase, high pressure processes or combinationsthereof for example.

Slurry phase processes (also referred to as particle formpolymerization) generally include forming a suspension of solid,particulate polymer in a liquid polymerization medium, to which monomersand optionally hydrogen, along with catalyst, are added. The suspension(which may include diluents) may be intermittently or continuouslyremoved from the reactor where the volatile components can be separatedfrom the polymer and recycled, optionally after a distillation, to thereactor. The liquefied diluent employed in the polymerization medium maybe a diluent for the solid polymer particles that is separate from andin addition to the unreacted monomers. Suitable diluents included thoseknown in the art and include hydrocarbons which are inert and liquid orare super critical fluids under slurry polymerization conditions. Forexample, suitable diluents include isobutene, propane, n-pentane,i-pentane, neopentane and n-hexane. Alternatively, the liquid medium maybe the unreacted monomer itself. A bulk phase process is similar to thatof a slurry process with the exception that the liquid medium is alsothe reactant (e.g., monomer) in a bulk phase process. However, a processmay be a bulk process, a slurry process or a bulk slurry process, forexample.

In a specific embodiment, a slurry process or a bulk process may becarried out continuously in one or more loop reactors. In continuousloop reactors, feed materials, such as monomer and catalyst areintroduced to the reactor and a product slurry containing solidpolyolefin particles in the liquid medium is taken off. In continuousloop reactors, the various feed materials may be introduced to the loopreaction zone in various ways. For example, the monomer and catalyst maybe introduced separately or together and the monomer and catalyst may bemixed with varying amounts of diluent prior to introduction to thereaction zone. In the loop reaction zone, the monomer and catalystbecome dispersed in the fluid slurry. As they circulate through the loopreaction zone in the fluid slurry, the monomer reacts at the catalystsite in a polymerization reaction and the polymerization reaction yieldssolid polyolefin particles in the fluid slurry. The loop reactor may bemaintained at a pressure of from about 27 bar to about 50 bar or fromabout 35 bar to about 45 bar and a temperature of from about 38° C. toabout 121° C., for example. Depending on the selection of diluent,monomer and optional comonomer, the reactor may also be operated atsuper-critical conditions. Reaction heat may be removed through the loopwall via any suitable method, such as via a double-jacketed pipe or heatexchanger, for example. Additional details regarding loop reactorapparatus and polymerization processes may be found, for example, inU.S. Pat. No. 4,674,290, U.S. Pat. No. 5,183,866, U.S. Pat. No.5,455,314, U.S. Pat. No. 5,565,174, U.S. Pat. No. 6,045,661, U.S. Pat.No. 6,051,631, U.S. Pat. No. 6,114,501, and U.S. Pat. No. 6,262,191,which are incorporated in their entirety herein.

Alternatively, other types of polymerization processes may be used, suchas stirred reactors in series, parallel or combinations thereof, forexample. Upon removal from the reactor, the polyolefin may be passed toa polymer recovery system for further processing, such as addition ofadditives and/or extrusion, for example. Such processes are known to oneskilled in the art and therefore are not described in detail herein.

The olefin monomers utilized in the processes described herein may beselected from C₂ to C₃₀ olefin monomers, or C₂ to C₁₂ olefin monomers(e.g., ethylene, propylene, butene, pentene, 4-methyl-1-pentene, hexene,octene and decene), for example. The monomers may include olefinicunsaturated monomers, C₄ to C₁₈ diolefins, conjugated or nonconjugateddienes, polyenes, vinyl monomers and cyclic olefins, for example.Non-limiting examples of other monomers may include norbornene,norbornadiene, isobutylene, isoprene, vinylbenzycyclobutane, styrene,alkyl substituted styrene, ethylidene norbornene, dicyclopentadiene andcyclopentene, for example. The formed polyolefin may includehomopolymers, copolymers or terpolymers, for example. In one or moreembodiments, the olefin monomers are selected from C₂-C₃ olefinmonomers.

In one or more embodiments, the processes described herein include thehomopolymerization of ethylene. In alternative embodiments, theprocesses described herein include the copolymerization of ethylene anda higher 1-olefin, such as butene, 1-pentene, 1-hexene, 1-octene or1-decene, for example. For example, the process may include thecopolymerization of ethylene and a starting amount of comonomer rangingfrom 0.01 wt. % to 10 wt. %, or from 0.01 wt. % to 5 wt. %, or from 0.1wt. % to 4 wt. % (wherein the resulting copolymers may still be referredto as polyethylene).

The polyolefins (and blends thereof) formed via the processes describedherein may include, but are not limited to, linear low densitypolyethylene (LLDPE), low density linear polyethylene (LDLPE),elastomers, plastomers, high density polyethylenes (HDPE), low densitypolyethylenes (LDPE), medium density polyethylenes (MDPE), polypropyleneand polypropylene copolymers, for example.

One or more embodiments include polymerizing an olefin monomer in aplurality of reaction zones under polymerization conditions to formmulti-modal polyolefins. Embodiments described herein are uniquelycapable of forming and maintaining the production of multi-modalpolyolefins having a desired set of characteristics. The desired set ofcharacteristics can include any of a variety of properties, includingbut not limited to, density and melt index, for example.

A single composition including a plurality of molecular weight peaks isconsidered to be a “multi-modal” polyolefin. Multi-modal polyolefins maybe produced via a variety of processes, such as polymerization processesutilizing multi-modal catalyst systems (i.e., catalyst systems includingat least two different catalytically active metal components). However,embodiments described herein employ at least two reaction zones, eachhaving its own set of polymerization conditions, to form the multi-modalpolyolefins. The reaction zones may be connected in series, such that atransfer effluent from a reaction zone, such as a first reaction zone,is transferred to a subsequent reaction zone, such as a second reactionzone, and so forth, until the multi-modal polyolefin product isdischarged from a final reaction zone with the desired set ofcharacteristics.

One or more embodiments include processes of forming bimodalpolyolefins. As used herein, the term “bimodal polyolefin” refers to asingle polyolefin composition including at least one identifiable highmolecular weight fraction and at least one identifiable low molecularweight fraction. Accordingly, such embodiments utilize a first reactionzone connected in series to a second reaction zone so that the transfereffluent withdrawn from the first reaction zone (which generallyincludes a first polyolefin and unreacted olefin monomer) is introducedinto the second reaction zone and a second reactor product formed in thesecond reaction zone is withdrawn therefrom and includes the bimodalpolyolefin. In the preparation of bimodal polyolefins, the highmolecular weight fraction and the low molecular weight fraction can beprepared in any order in the reaction zones, e.g., the low molecularweight fraction may be formed in a first reaction zone and the highmolecular weight fraction in a second reaction zone, or vice versa, forexample.

A comonomer that varies from the olefin monomer but is selected from,the same components may also be introduced into the second reactionzone. In one or more embodiments, the comonomer is selected from thosedescribed previously herein. For example, the comonomer may be selectedfrom C₄-C₈ olefin monomers. In one or more specific embodiments, thecomonomer may include butene or hexene.

The first reaction zone is generally operated under first polymerizationconditions while the second reaction zone is generally operated undersecond polymerization conditions. The first polymerization conditionsand the second polymerization conditions will be adapted to formpolyolefins having the desired set of characteristics. As such, thefirst polymerization conditions and the second polymerization conditionsmay vary from one another. However, it is contemplated that in certaincircumstances the first and second polymerization conditions may besimilar, if not the same. For example, in one or more embodiments, thesame catalyst system is utilized in the plurality of reaction zones.However, in other embodiments, different catalyst systems are used inthe plurality of reaction zones.

In one or more embodiments, the reaction zones are independentlyselected from loop reactors. In one or more specific embodiments, eachreaction zone is a loop reactor. As stated previously herein, while thepresent discussion may primarily focus on two reaction zones in series,the present techniques may be applicable to more than two reaction zonesin series. The loop reactors may or may not have the same dimensions,including but not limited to volume, length, diameter, height, number ofreactor segments, configuration of the layout in the vertical andhorizontal directions and others, for example.

In one or more embodiments, the polymerization process includesseparation. The separation can occur at any point within the process.For example, separation may occur after withdrawing the second reactorproduct from the second reaction zone. Alternatively (in combinationtherewith), the process may include separating the first reactorproduct, either within the transfer effluent or another stream withdrawnfrom the first reaction zone. Such separation can be accomplished bymethods known in the art and may include, without limitation,concentration devices, such as hydrocyclones, flashing devices andcombinations thereof, for example. Such processes are known to oneskilled in the art and therefore are not described in detail herein.

One or more specific embodiments include separating at least a portionof the transfer agent to form a lighter stream and a heavier stream. Theheavier stream is then introduced into the second reaction zone.Alternative embodiments include withdrawing a second effluent from thefirst reaction zone, which is then separated to form a lighter streamand a heavier stream, while the transfer effluent may then be introduceddirectly to the second reaction zone (without separation), oralternatively may undergo separation prior to introduction into thesecond reaction zone as well.

The characteristics/properties of polyolefins produced in polymerizationprocess are a function of at least reaction zone conditions and theratio of comonomer:olefin monomer. Process variables in a reaction zonecan change suddenly and their effect on the olefin monomer concentrationin the reaction zone may not be detected by measurement technology. Thetables below illustrate the results of simulations wherein variousprocess conditions change and the effect on the feed rate (e.g.,comonomer:olefin monomer concentration in the second reaction zone) inthe absence of quick control and correction.

FIG. 1 illustrates a simplified schematic polymerization process for usein the following simulations. Ethylene is introduced in Stream A to afirst reaction zone, Reactor 1, under first polymerization conditions(95° C., 615 psia, 120-200 ppm cocatalyst (TEAl, TEB, TiBAl), 3-4 wt. %C₂, 1-5 mol % H₂) to form polyethylene (HDPE, <0.3 HLMI, <0.927 g/ccdensity) present in transfer effluent, which is transferred to a secondreaction zone, Reactor 2, under second polymerization conditions (85°C., 615 psia, 300-400 ppm cocatalyst (TEAl, TEB, TiBAl), 0.5-1.5 wt. %C₂, 2-4 wt. % C₆) to form the second reactor product including thebimodal polyethylene (LLDPE, >50 MI, >0.960 g/cc density). Comonomerhexene is introduced into the second reaction zone, Reactor 2, as wellas additional olefin monomer.

TABLE 1 The initial ethylene feed rate to reaction zone 1 remainedconstant Feed Feed Stream rate (lb/hr) Feed rate (lb/hr) rate (lb/hr)Olefin monomer 100,000 100,000 100,000 Overflow ethylene 6,000 1,0001,000 Additional monomer 94,000 94,000 99,000 Comonomer 3,000 3,0003,000 Wt. Ratio of 0.030 0.032 0.30 comonomer:ethylene in R2 % change5.3

In Table 1, as the rate of reaction in reaction zone 1 increased, lessoverflow ethylene was introduced into reaction zone 2. Table 1 reflectsa situation where the comonomer introduction does not timely adjust tothe change in overflow ethylene feed rate, resulting in the polyolefinnot meeting predetermined characteristics.

TABLE 2 The initial ethylene feed rate to reaction zone 1 remainedconstant Feed Stream Feed rate (lb/hr) Feed rate (lb/hr) rate (lb/hr)Olefin monomer 100,000 75,000 75,000 Overflow ethylene 4,000 3,000 3,000Additional monomer 96,000 72,000 72,000 Comonomer 3,000 3,000 2,500Ratio of 0.030 0.040 0.30 comonomer:ethylene in R2 % change 33.3

In Table 2, as the introduction of olefin monomer into reaction zone 1is interrupted, the overflow ethylene concentration drops and a lagoccurs before comonomer and monomer introduction is adjusted, resultingin polyolefin not meeting predetermined characteristics.

TABLE 3 The initial ethylene feed rate to reaction zone 1 remainedconstant Feed Feed Stream rate (lb/hr) Feed rate (lb/hr) rate (lb/hr)Olefin monomer 100,000 100,000 100,000 Overflow ethylene 4,000 20,00020,000 Additional monomer 96,000 96,000 80,000 Comonomer 3,000 3,0002,500 Ratio of 0.030 0.026 0.30 comonomer:ethylene in R2 % change −13.8

In Table 3, as the reaction rate of reaction zone 1 decreased, theoverflow ethylene increased and a lag was observed before comonomerintroduction was adjusted, resulting in polyolefin not meetingpredetermined characteristics.

Unfortunately, it can be difficult to measure polymer characteristics,such as properties, in the process or at intermediated stages thereof inreal time. Accordingly, a delay in process adjustments as a result ofsuch changing polymer characteristics can cause less than desiredreaction production and polyolefin property control. However, real time,on-line prediction and control of monomer concentration within apolymerization process are substantially improved utilizing theembodiments of the invention.

As stated previously herein, embodiments described herein are uniquelycapable of forming and maintaining the production of multi-modalpolyolefins having a desired set of characteristics. Accordingly, one ormore embodiments include maintaining an essentially constantcomonomer:olefin monomer ratio (i.e., the “ratio”) in the secondreaction zone (when referring to a bimodal system and which may beaccordingly adjusted when referring to systems having more than tworeaction zones). As used herein, the term “essentially constant” refersto a ratio or property that varies by not more than 20%, or not morethan 10%, or not more than 5%, or not more than 2%, under standardoperating conditions. For example, the comonomer:olefin monomer weightratio in the second reaction zone may be maintained at a ratio of from0.005:1 to 100:1 or from 0.5:100 to 100:1, or from 1:1 to 10:1, or from1.33:1 to 8:1, or from 1.5:1 to 5:1, for example.

The variance of the comonomer:olefin monomer ratio in the secondreaction zone may be calculated by a variety of methods, including, butnot limited to: (1) the difference between the highest and lowest ratios“under standard operating conditions” when divided by the lowest ratio,(2) the difference between the highest and lowest ratios “under standardoperating conditions” when divided by the highest ratio, (3) thedifference between the highest and lowest ratios “under standardoperating conditions” when divided by an average of the lowest andhighest ratios, or (4) the ratio varies from a target set point. Forexample, the variance in the comonomer:olefin monomer ratio in thesecond reactor when utilizing the 4^(th) type of calculation wherein thetarget set point is the ratio at the start of the polymerization processand is 1:1, for example and the ratio at a later time is 0.75:1 resultsin a variance of 25%.

As used in the present embodiments, the olefin monomer is the olefinwith the highest molar concentration in the polyolefin while thecomonomer is any olefin whose molar concentration in the polymer is lessthan that of the olefin monomer. Although the calculations anddiscussion have focused on a monomer/single comonomer system, theteachings of this disclosure are equally applicable to amonomer/multiple comonomer system.

As with other process conditions and polymer properties, there are fewpractical ways to measure the comonomer:olefin monomer ratio directly.However, the ratio can be determined from a variety of methods, such asanalysis/spectroscopy or material or energy or heat balancecalculations, for example. Currently, the feed rates or operatingconditions that affect the ratio are adjusted manually based on apolymerization rate determined by assuming a constant conversion. Thismethod for determining the polymerization rate may be adequate for longterm (several reactor residence times) and average out because lossesare constant and unreacted ethylene is recycled. However, it does notadequately reflect short term fluctuations in the polymerization rate.

In one or more embodiments, maintaining the essentially constantcomonomer:olefin monomer ratio in the second reaction zone includesdetermining a concentration of carry over olefin monomer in the transfereffluent and adjusting a rate of introduction of the additional olefinmonomer into the second reaction zone, adjusting a rate of introductionof the comonomer into the second reaction zone or a combination thereofin response to the carry over olefin monomer concentration.

The determination of the carry over olefin monomer concentration may bedetermined by an analysis method, such as Ramen spectroscopy. Forexample, in one or more embodiments, the carry over olefin monomerconcentration is determined by a process that includes irradiatingin-situ the transfer effluent, measuring scattered energy from thetransfer effluent and determining from the measured scattered energy thecarry over olefin monomer concentration.

In one or more embodiments, the carry over olefin monomer concentrationis determined by calculating the production rate of first polyolefin inthe first reaction zone. The production rate (PR) of the firstpolyolefin in the first reaction zone may be determined from thecalculated reaction quotient (Q_(rxn)) and the heat of polymerizationper unit of polyolefin produced (i.e., PR=Q_(rxn)/ΔH_(rxn)). Thecalculated reaction quotient can be calculated from known equationsand/or programs which may include the following equations:Q _(rxn) =Q _(TOT) −Q _(no rxn)Q _(rxn) =WCp(ΔT _(TOT) −T _(no rxn))

In one or more embodiments, the carry over olefin monomer concentrationis determined by performing an energy balance calculation for the firstreaction zone. Such calculation can be done by an on-line computerprogram utilizing the energy balance around the reactor. The program canthen calculate the feed rate required to maintain the desired propertiesand automatically adjust, the set point of the flow controller for thefeed.

While the embodiments described herein are capable of maintaining anessentially constant comonomer:olefin monomer ratio in the secondreaction zone, it is noted that the concentration of various componentswithin the slurry may vary as the slurry flows around the loop reactorand some of the components are consumed by reaction. For example, in anillustrative 68137 liter (18,000 gallon) loop reactor being used for theslurry polymerization of ethylene, there are approximately 48,000 pounds(about 18,000 kg) of liquid with approximately 2,200 pounds (about 800kg) of ethylene in the liquid. At a production rate of approximately40,000 lbs/hr (about 15,000 kg/hr), the process consumes approximately333 lbs (about 125 kg) of ethylene in the time it takes to flow aroundthe reactor loop. The ethylene concentration in the loop is calculatedto range between about 4.27 wt. % just before the ethylene feed locationto about 4.93 wt. % just after the ethylene feed location.

One or more embodiments include a plurality of olefin monomer feedlocations to the second reaction zone. In addition (or alternativethereto), the second reaction zone may include a plurality of comonomerfeed locations, hydrogen feed locations or combinations thereof. It iscontemplated that the first reaction zone may also include a pluralityof olefin monomer feed locations. Alternatively, the first reaction zonemay utilize a single olefin monomer feed location. As used herein, theterm “single feed, location” refers to a feed location for a particularfeed component, such as olefin monomer or comonomer, for example, and isnot intended to limit the feed of separate components to that singlefeed location nor prohibit the feed of multiple components to thatsingle feed location.

The plurality of feed locations can be utilized to feed components tothe reaction zone at any location sufficient to maintain the desiredcontinuity of slurry concentration throughout. Furthermore, theparticular placement of each feed location will depend upon systemspecifics but will be selected so as to maintain the essentiallyconstant olefin monomer:comonomer ratio, in one or more embodiments, theplurality of feed locations are spaced essentially equidistant over thelength of the reactor.

In one or more embodiments, the plurality of feed locations includes twofeed locations. In another embodiment, the plurality of feed locationsincludes more than two feed locations. For example, the plurality offeed locations may include 3, 4, 5 or 6 feed locations. Furthermore, thenumber of feed locations for each component need not be the same. Forexample, the second reaction zone may include 2 olefin monomer feedlocations and a single comonomer feed location. Alternatively, thesecond reaction zone may include 4 olefin monomer feed locations and 2comonomer feed locations or vice versa. However, it is contemplated thatthe second reaction zone may include the same number of feed locationsfor each component, such as 2 olefin monomer feed locations and 2comonomer feed locations, for example.

The concentration of feed introduced at each feed location will be suchas required to maintain the essentially constant monomer:comonomerratio, as discussed previously herein. Such amount may be essentiallythe same at each feed location or may vary by an amount, such as notless than 20%, or less than 10%, or less than 5%, or less than 2%, forexample.

FIG. 2 illustrates an embodiment of a single loop reactor that can beutilized as the second reaction zone (and/or the first in particularembodiments) wherein a plurality of feed locations are utilized for avariety of the slurry components. In FIG. 2, the loop reactor 10includes major segments 12, upper minor segments 14 and lower minorsegments 16. The minor segments may simply be curved elbows that jointhe major segment and may be relatively curved to facilitate continuoustake-off of product slurry. In FIG. 2, the loop reactor 10 has eightmajor segments, although it is contemplated that the present process andapparatus may be used with a loop reactor having a higher or lowernumber of major segments, for example, a loop reactor having four legsor twelve segments. It will further be understood that the particularnumbering of segments herein does not necessarily imply a priority tothe legs, as the loop reactor is circular. The legs are surrounded with,cooling jackets 18 for heat exchange. Each segment or leg is connectedto the next segment or leg by a smooth bend or elbow 20, thus providinga continuous flow path substantially free from internal obstructions.The fluid slurry is circulated by means of an impeller (not shown)driven by motor 24.

Monomer (which may be mixed with a diluent) is supplied to the reactor10 through two monomer feeds (illustrated as the connection of conduit30 to the loop reactor). Comonomer may be introduced via conduit 30 orvia another feed location. Catalyst is introduced, via conduits tocatalyst feeds 44 which each provide a zone for catalyst introduction.In the embodiments illustrated in FIG. 2, the catalyst feeds 44 are alsosymmetrically arranged around the reactor. The loop reactor furtherincludes means for removing a portion of the slurry from the reactor(product take-offs). The means for removing the slurry portion may be asettling let, a hollow appendage for continuous take-off, or anotherconduit for removing the product slurry without substantial leakage orinterference with the loop reactor operation. In FIG. 2, elongatedhollow appendages for continuously taking-off an intermediate productslurry are designated by reference character 34. Continuous take-offmechanism 34 is located in or adjacent to one of the lower horizontalreactor loop sections 16, and/or adjacent or on a connecting elbow 20.

It has been determined that specific polyolefin properties, and inparticular polyolefin density, can be controlled as a function of meltindex. In particular, it has been determined that when formingmulti-modal polyolefins, the multi-modal polymer density can becontrolled by controlling the melt index of the first polyolefin (i.e.,the polyolefin present in the transfer effluent withdrawn from the firstreaction zone). This may be in combination with or apart frommaintaining an essentially constant comonomer:olefin monomer ratio inthe second reaction zone as discussed previously herein.

The melt index of the first polyolefin can be measured via methods knownin the art. Alternatively, the melt index of the first polyethylene canbe measured by use of Ramen spectroscopy. For example, a Ramen probe maybe inserted into the transfer effluent to measure the melt index of thefirst polyethylene present therein to provide real time measurement ofthe melt index and any resulting adjustments for control thereof.Alternatively, samples may be drawn from the transfer effluent toanalyze and determine the melt index of the first polyethylene.

Alternatively to the transfer effluent, it is contemplated that whenseparation processes are utilized with the first reaction zone product(as described previously herein), the melt index of the firstpolyethylene can be measured in the heavier stream (which may or may notbe introduced into the second reaction zone depending on whether theseparation occurs in a second effluent or the transfer effluent).

To further illustrate determination of the melt index and densitycorrelation, sample calculations are included below as well asillustrations correlating density of the first fraction (i.e., the firstpolyethylene) and the second fraction (formed in the second reactionzone) as well as the final bimodal polyethylene density.

TABLE 4 Density correlation between polymer fractions Fraction FractionFinal Fraction 1 % Fraction 2 % 1 density 2 density product density 0.50.5 0.9600 0.9250 0.9422 0.5 0.5 0.9650 0.9250 0.9446 0.5 0.5 0.97000.9250 0.9470 0.5 0.5 0.9750 0.9250 0.9493 0.4 0.6 0.9600 0.9250 0.93870.4 0.6 0.9650 0.9250 0.9406 0.4 0.6 0.9700 0.9250 0.9425 0.4 0.6 0.97500.9250 0.9444 0.6 0.4 0.9600 0.9250 0.9457 0.6 0.4 0.9650 0.9250 0.94860.6 0.4 0.9700 0.9250 0.9515 0.6 0.4 0.9750 0.9250 0.9544

As shown in Table 4, the density in Fraction 2 remains essentiallyconstant (as desired and discussed previously herein with regards tomaintaining an essentially comonomer:olefin monomer ratio). Accordingly,the Fraction 1 density is adjusted when a change in final productdensity is desired.

FIGS. 3 and 4 illustrate the correlation between melt index and densityfor 2 different, polymer systems. When provided with 3 polymer samplesformed in a system that have been analyzed for melt index and density, aregression may be undertaken on those 3 samples to determine thecorrelation between melt index and density. For example, the correlationbetween melt index and density for the essentially linear systemillustrated in FIG. 3 is M1=10498*density-9973.1. Accordingly, once acorrelation is made between the melt index and density (e.g., thedensity of the desired fraction, such as Fraction 1), the target meltindex for that fraction can be correlated, measured and controlled toresult in a final product having a pre-determined target density.

Control may be accomplished via methods known in the art, such asadjusting the hydrogen:olefin monomer ratio to the first reaction zone.Further, it is to be understood that the relationship between melt indexand density can vary by system but one skilled in the art can determinethe response based on the techniques described herein without undueexperimentation.

In one or more specific embodiments, the multi-modal polyolefin is anethylene based polymer. Unless otherwise specified, the term “ethylenebased polymer” refers to polyethylene having at least about 50 wt. %, orat least about 80 wt. %, or at least about 85 wt. %, or at least about90 wt. % or at least about 95 wt. % polyethylene based on the totalweight of polymer. Accordingly, in one or more embodiments, the bimodalpolyolefin includes an ethylene based polymer. For example, the bimodalethylene based polymer may include a low molecular weight high densityfraction (produced in one reaction zone) and a high molecular weightlinear low density fraction (produced in the other reaction zone).

In one or more embodiments, the high molecular weight fraction exhibitsa molecular weight that is greater than the molecular weight of the lowmolecular weight fraction. The high molecular weight fraction may have amolecular weight of greater than 100,000, for example. In contrast, thelow molecular weight fraction may have a molecular weight of from 15,000to 50,000, for example. The bimodal polyolefin may include more highmolecular weight fraction than low molecular weight fraction. Forexample, the bimodal polyolefin may include greater than 50 wt. % highmolecular weight fraction, for example. Alternatively, the bimodalpolyolefin may include less high molecular weight fraction than lowmolecular weight fraction. For example, the bimodal polyolefin mayinclude less than 50 wt. % high molecular weight fraction, for example.The average molecular weight of the bimodal polyolefin (or a fractionthereof) is herein generally referred to as “molecular weight”. Inpractice, the average molecular weight of the bimodal polyolefin may bethe number average, weight average, viscosity average, z average z+1average, as well as other average characterizations.

Alternatively, the various fractions of the bimodal polyethylene may bereferred to as a first polyethylene fraction and a second polyethylenefraction. The high molecular weight fraction and the low molecularweight fraction may be the same as either the first or secondpolyethylene fraction depending upon the polymerization conditions.However, the first polyethylene fraction is formed in the first reactionzone while the second reaction zone is formed in the second reactionzone. In one or more embodiments, the first polyethylene fraction is thelow molecular weight fraction, while the second polyethylene fraction isthe high molecular weight fraction. In one or more specific embodiments,the bimodal polyethylene includes at least 40% first polyethylenefraction. In other embodiments, the bimodal polyethylene includes from40$ to 60% first polyethylene fraction.

Each fraction of the bimodal polyethylene may have a density (asmeasured by ASTM D-792) of from about 0.86 g/cc to about 0.98 g/cc, orfrom about 0.88 g/cc to about 0.965 g/cc, or from about 0.90 g/cc toabout 0.965 g/cc or from about 0.925 g/cc to about 0.97 g/cc, forexample.

In one or more embodiments, one or more of the fractions may includehigh density polyethylene. As used herein, the term “high densitypolyethylene” refers to ethylene based polymers having a density of fromabout 0.94 g/cc to about 0.97 g/cc, for example.

In one or more embodiments, one or more of the fractions may include lowdensity polyethylene. As used herein, the term “low densitypolyethylene” refers to ethylene based polymers having a density of lessthan about 0.92 g/cc, for example.

The polyolefins and blends thereof are useful in applications known toone skilled in the art, such as forming operations (e.g., film, sheet,pipe and fiber extrusion and co-extrusion, as well as blow molding,injection molding and rotary molding). Films include blown, oriented orcast films formed by extrusion or co-extrusion or by lamination usefulas shrink film, cling film, stretch film, sealing films, oriented films,snack packaging, heavy duty bags, grocery sacks, baked and frozen foodpackaging, medical packaging, industrial liners, and membranes, forexample, in food-contact and non-food contact application. Fibersinclude slit-films, monofilaments, melt spinning, solution spinning andmelt blown fiber operations for use in woven or non-woven form to makesacks, bags, rope, twine, carpet backing, carpet yarns, filters, diaperfabrics, medical garments and geotextiles, for example. Extrudedarticles include medical tubing, wire and cable coatings, sheets, suchas thermoformed sheets (including profiles and plastic corrugatedcardboard), geomembranes and pond liners, for example. Molded articlesinclude single and multi-layered constructions in the form of bottles,tanks, large hollow articles, rigid food containers and toys, forexample.

CLOSING OF THE DETAILED DESCRIPTION

Therefore, the present invention is well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Theparticular embodiments disclosed above are illustrative only, as thepresent invention may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. Furthermore, no limitations axeintended to the details of construction or design herein shown, otherthan as described in the claims below. It is therefore evident that, theparticular illustrative embodiments disclosed above may be altered,combined, or modified and all such variations are considered within thescope and spirit of the present invention.

The invention illustratively disclosed herein suitably may be practicedin the absence of any element that is not specifically disclosed hereinand/or any optional element disclosed herein. While compositions andmethods are described in terms of “comprising,” “containing,” or“including” various components or steps, the compositions and methodscan also “consist essentially of” or “consist of” the various componentsand steps. All numbers and ranges disclosed above may vary by someamount. Whenever a numerical range with a lower limit and an upper limitis disclosed, any number and any included range falling within the rangeare specifically disclosed. In particular, every range of values (of theform, “from about a to about b,” or, equivalently, “from approximately ato b,” or, equivalently, “from approximately a-b”) disclosed herein isto be understood to set forth every number and range encompassed withinthe broader range of values.

This concludes the detailed description. The particular embodimentsdisclosed above are illustrative only, as the invention may be modifiedand practiced in different but equivalent manners apparent to thoseskilled in the art having the benefit of the teachings herein.Furthermore, no limitations are intended to the details of constructionor design herein shown, other than as described in the claims below. Itis therefore evident that the particular embodiments disclosed above maybe altered or modified and all such variations are considered within thescope and spirit of the invention. Accordingly, the protection soughtherein is as set forth in the claims below.

What is claimed is:
 1. A process of forming polyolefins comprising:introducing olefin monomer selected from C₂-C₃ olefins into a firstreaction zone under first polymerization conditions to form a firstpolyolefin; withdrawing a transfer effluent from the first reactionzone, the transfer effluent comprising first polyolefin and unreactedolefin monomer; introducing the transfer effluent, a comonomer selectedfrom C₄-C₈ olefins, and additional olefin monomer to a second reactionzone under second polymerization conditions to form a second reactorproduct; maintaining an essentially constant comonomer:olefin monomerratio in the second reaction zone; and withdrawing at least a portion ofthe second reactor product, wherein the second reactor product comprisesa bimodal polyolefin.
 2. The process of claim 1, wherein the olefinmonomer comprises ethylene.
 3. The process of claim 2, wherein thecomonomer comprises hexene.
 4. The process of claim 1, wherein the firstreaction zone, the second reaction zone or a combination thereofcomprise a loop slurry reaction vessel.
 5. The process of claim 1,wherein the second reaction zone comprises a loop slurry reaction vesseland the loop slurry reaction vessel comprises a plurality of olefin feedlocations, comonomer feed locations, hydrogen feed locations, or acombination thereof.
 6. The process of claim 1, wherein maintaining acomonomer:olefin monomer ratio essentially constant comprisesdetermining a concentration of carry over olefin monomer in the transfereffluent; and adjusting a rate of introduction of the additional olefinmonomer into the second reaction zone, adjusting a rate of introductionof the comonomer into the second reaction zone or a combination thereofin response to the carry over olefin monomer concentration.
 7. Theprocess of claim 6, wherein the carry over olefin monomer concentrationis determined by a process comprising: irradiating in-situ the transfereffluent; measuring scattered energy from the transfer effluent; anddetermining from the measured scattered energy the carry over olefinmonomer concentration.
 8. The process of claim 6, wherein the carry overolefin monomer concentration is determined by calculating the productionrate of first polyolefin in the first reaction zone.
 9. The process ofclaim 8, wherein the production rate of the first polyolefin in thefirst reaction zone is determined from the calculated reaction quotient(Q_(rxn)) and the heat of polymerization per unit of polyolefinproduced.
 10. The process of claim 6, wherein the carry over olefinmonomer concentration is determined by performing an energy balancecalculation for the first reaction zone.
 11. The process of claim 1,wherein the bimodal polyolefin comprises a first polyethylene fractionhaving an average molecular weight of from 15,000 to 50,000 and a secondpolyethylene fraction having an average molecular weight of greater than100,000.
 12. The process of claim 11, wherein the bimodal polyolefincomprises at least 40% first polyethylene fraction.
 13. The process ofclaim 11, wherein the bimodal polyolefin comprises from 40% to 60% ofthe first polyethylene fraction.
 14. A process of forming polyolefinscomprising: introducing olefin monomer selected from C₂-C₃ olefins andhydrogen into a first reaction zone under first polymerizationconditions to form a first polyolefin; withdrawing a transfer effluentfrom the first reaction zone, the transfer effluent comprising firstpolyolefin and unreacted olefin monomer; introducing the transfereffluent, a comonomer selected from C₄-C₈ olefins, and additional olefinmonomer to a second reaction zone under second polymerization conditionsto form a second reactor product; determining a melt index of the firstpolyolefin in the transfer effluent, the first reaction zone or acombination thereof; correlating density of the first polyolefin withthe melt index of the first polyolefin; and adjusting a rate ofintroduction of the hydrogen into the first reaction zone in response tothe melt index of the first polyolefin and a pre-determined bimodalpolyolefin density; and withdrawing at least a portion of the secondreactor product, wherein the second reactor product comprises thebimodal polyolefin.
 15. The process of claim 14 further comprisingseparating at least a portion of the transfer effluent to form a lighterstream and a heavier stream; and determining a melt index of the firstpolyolefin in the heavier stream.
 16. The process of claim 14, whereinthe heavier stream is introduced into the second reaction zone.
 17. Theprocess of claim 15, wherein the separating comprises passing the atleast a portion of the transfer effluent through a flash tank, ahydrocyclone or a combination thereof to form the lighter stream and theheavier stream.
 18. A process of forming polyolefins comprising:introducing olefin monomer selected from C₂-C₃ olefins and hydrogen intoa first reaction zone under first polymerization conditions to form afirst polyolefin; withdrawing a transfer effluent from the firstreaction zone, the transfer effluent comprising first polyolefin andunreacted olefin monomer; withdrawing a second effluent from the firstreaction zone; separating at least a portion of the second effluent toform a lighter stream and a heavier stream; introducing the transfereffluent, a comonomer selected from C₄-C₈ olefins, and additional olefinmonomer to a second reaction zone under second polymerization conditionsto form a second reactor product; determining a melt index of the firstpolyolefin in the heavier stream; correlating density of the firstpolyolefin with the melt index of the first polyolefin; and adjusting arate of introduction of the hydrogen into the first reaction zone inresponse to the melt index of the first polyolefin and a pre-determinedbimodal polyolefin density; and withdrawing at least a portion of thesecond reactor product, wherein the second reactor product comprises thebimodal polyolefin.
 19. The process of claim 18, wherein the separatingcomprises passing the at least a portion of the transfer effluentthrough a flash tank, a hydrocyclone or a combination thereof to formthe lighter stream and the heavier stream.
 20. A process of controllingbimodal polyolefin density, the process comprising: introducing olefinmonomer selected from C₂-C₃ olefins and hydrogen into a first reactionzone under first polymerization conditions to form a first polyolefinexhibiting a first density; withdrawing a transfer effluent from thefirst reaction zone, the transfer effluent comprising first polyolefinand unreacted olefin monomer; introducing the transfer effluent, acomonomer selected from C₄-C₈ olefins, and additional olefin monomer toa second reaction zone under second polymerization conditions to form asecond polyolefin exhibiting a second density; withdrawing at least aportion of a second reactor product from the second reaction zone,wherein the second reactor product comprises a bimodal polyolefincomprising the first polyolefin and the second polyolefin and exhibitinga bimodal polyolefin density; controlling the bimodal polyolefin densitywithin a target density by a process comprising: maintaining anessentially constant second density within the second reaction zone bymaintaining an essentially constant comonomer:olefin monomer ratio inthe second reaction zone; determining a melt index of the firstpolyolefin in the transfer effluent, the first reaction zone or acombination thereof; correlating the first density with the melt indexof the first polyolefin; and adjusting a rate of introduction of thehydrogen into the first reaction zone in response to the melt index ofthe first polyolefin and the target density.
 21. The process of claim20, wherein maintaining a comonomer:olefin monomer ratio essentiallyconstant comprises determining a concentration of carry over olefinmonomer in the transfer effluent; and adjusting a rate of introductionof the additional olefin monomer into the second reaction zone,adjusting a rate of introduction of the comonomer into the secondreaction zone or a combination thereof in response to the carry overolefin monomer concentration.
 22. The process of claim 21, wherein thecarry over olefin monomer concentration is determined by a processcomprising: irradiating in-situ the transfer effluent; measuringscattered energy from the transfer effluent; and determining from themeasured scattered energy the carry over olefin monomer concentration.23. The process of claim 21, wherein the carry over olefin monomerconcentration is determined by calculating the production rate of firstpolyolefin in the first reaction zone.
 24. The process of claim 23,wherein the production rate of the first polyolefin in the firstreaction zone is determined from the calculated reaction quotient(Q_(rxn)) and the heat of polymerization per unit of polyolefinproduced.
 25. The process of claim 21, wherein the carry over olefinmonomer concentration is determined by performing an energy balancecalculation for the first reaction zone.
 26. The process of claim 20,wherein the bimodal polyolefin comprises a first polyethylene fractionhaving an average molecular weight of from 15,000 to 50,000 and a secondpolyethylene fraction having an average molecular weight of greater than100,000.
 27. The process of claim 20, wherein the bimodal polyolefincomprises at least 40% first polyethylene fraction.